1. Field of the Invention
The present invention relates generally to hard copy document apparatus and, more particularly, to cut sheet print media automatic document feeders.
2. Description of Related Art
Automated business machines for producing or reproducing hard copy documents, such as copiers, printers, telecommunications facsimile machines, document scanners, and the like, are well known commercially. Ideally, when working with cut sheet print media, a copying apparatus often includes an automatic document feeder ("ADF") mechanism for automatically loading and unloading single sheet sequentially to a functional station where the copying apparatus performs an operation--e.g., sequentially scans the fed document sheets for copying, faxing, displaying on a computer monitor, or the like. Following the operation, the ADF then off-loads that sheet and feeds the immediately following sheet of the document to the functional station. A sequential flow of sheets by the ADF and positioning without the necessity of manual handling reduces the time required to accomplish the complete functional operation. Media which already contains printed matter (hereinafter referred to generically as a "document") presents a need for precise alignment to a scanning station in order to generate a true copy.
Two major problems associated with business machines that include an ADF are the occasional mis-feed, commonly known as a "paper jam," and a multiple sheet feed. Paper jams interrupt operation and require manual correction of the problem before the automated conveying can be restarted. Multiple sheet feed can result in a missing page in the copy.
Typical commercial ADFs generally require a document sheet pre-picking mechanism to ensure a single sheet gets to a pick roller, especially in systems having a horizontal input paper tray. Most commercial ADFs for scanners scroll the document page-by-page passed a stationary scan head and into an output tray. However, this makes prescanning and other multiple scanning operations difficult or even impossible, leading to degraded scan output quality.
Belt type document feeders have been adapted to place a document onto a flat, transparent, scanning bed. One such system is shown in U.S. Pat. No. 5,342,133 (Canfield), assigned to the common assignee of the present invention.
A beltless drive system is demonstrated in allowed U.S. patent application Ser. No. 08/651,066 (Hendrix), assigned to the common assignee of the present invention.
A third common problem is the inadvertent skewing of the document page in the loading and unloading of paper sheets on the glass of a scanner or copier. Skewing can often lead to a paper jam. A first prior art method is to affix a paper drive mechanism axle to the ADF structure and turn the axle with a motor and gear train or timing belt linkage. The motor and the linkage are also rigidly mounted to the ADF structure. The disadvantage of this method is that it is difficult to balance the normal force between the two drive rollers. The stiff elastomer durometer acts as a spring constant and any minor difference in the roller diameter, roller run-out, or roller position relative to the bed will result in a significant difference in the normal force between the two rollers. A second prior art method is to spring load an axle to the ADF structure, turning the end of the axle with a motor and gear train or timing belt linkage. The motor is rigidly mounted to the ADF structure. The linkage moves with the axle and pivots about the motor shaft. The disadvantage is that it becomes substantially impossible to balance the normal force between the two drive rollers. The linkage generates unequal force when moving the drive roller axle in a forward direction versus a reverse direction. Because the linkage is positioned at the end of the axle, this unequal force teeters the drive roller axle and results in uneven normal forces. A third prior art method is to again spring load the axle to the ADF structure, again turning the end of the axle with a motor and gear train or timing belt linkage with the motor and linage moving with the axle. Disadvantages are a difficulty in balancing the normal force between the two drive rollers and the susceptibility of a motor suspension to transportation shocks (the motor weight far exceeds the required drive roller normal force and therefore requires a complicated suspension to lift the motor on one end while maintaining a balance force to the other end of the axle. A fourth prior art method is to spring load an axle to the ADF structure and turn the axle in the middle. Both the motor and linkage move with the axle. The disadvantages of this method are that the motor weight far exceeds the required drive roller normal force and therefore requires a complicated suspension system in the middle of the ADF structure where it is geometrically constrained and that the motor suspension is susceptible to transportation shocks.
In order to have an ADF with a small workplace form factor (also sometimes referred to as a "desktop footprint"), it is desirable to input and output document sheets from vertically oriented trays. One vertically aligned paper sheet input mechanism is taught by Hock et al. in related U.S. Pat. Nos. 5,320,436 and 5,326,090, assigned to the common assignee of the present invention.
There is a need for an inexpensive ADF, having a simple paper path and being adaptable for hard copy apparatus that employ a flat bed document scanners.